Astronomers have used pulsating stars to trace the crooked shape of our galaxy’s disk.
A careful survey of more than 2,400 Cepheid variable stars has revealed the Milky Way’s warped disk in new detail. Dorota Skowron (University of Warsaw, Poland) and colleagues report the result in the August 2nd Science.
Cepheids are giants and supergiants that breathe in and out at a rate proportional to their intrinsic brightness. This period-luminosity relation makes them superb distance markers, as Henrietta Swan Leavitt discovered in the early 20th century.
Skowron used the Optical Gravitational Lensing Experiment (OGLE), as well as data from five other surveys and catalogs, to map the Cepheids’ 3D locations, which lie primarily within a few tens of thousands of light-years of the Sun. Their project confirms there’s a severe warp in our galaxy’s disk, reminiscent of pizza dough bent in its mid-toss flight. The warp has also shown up in maps using neutral hydrogen gas, stars, dust, and stellar motions, as well as a recent infrared study that used roughly half as many Cepheids as Skowron’s team did.
When they plotted the variable stars’ locations looking down at our galaxy’s disk, the astronomers noticed that the Cepheids clump, gathering in several concentrations that trace out a sloppy spiral pattern. Curious, the team took the three most prominent clumps and calculated the ages of the stars in them. They found that the stars in each group had a similar age to one another — approximately 64, 113, and 175 million years. The youngest clump’s stars tightly clustered together, whereas the oldest clump's stars were the most spread out.
The team thinks that these Cepheid populations were born in three bursts of star formation. As time passed, stars that formed together would have naturally gone their separate ways, explaining why the oldest stars are the most spread out. Computer simulations confirm that three starbirth episodes would have stretched into the pattern the team’s map reveals in the Milky Way.
Since all three rounds of star formation happened on the same side of the galaxy, Skowron speculates that an encounter with a dwarf galaxy might have triggered them.
Astronomers have reconstructed the Milky Way's 3D structure based on the precisely measured distances of more than 2,400 Cepheids (colored dots). The Sun icon marks our position.
J. Skowron / OGLE / Astronomical Observatory, University of Warsaw
Reference: D. M. Skowron et al. “A Three-Dimensional map of the Milky Way Using Classical Cepheid Variable Stars.” Science. August 2, 2019.